This invention relates to a hardness tester and more particularly to a constant depression, variable force hardness tester.
Prior hardness testing devices generally comprise constant force, variable depression, indentation type hardness testers. Such devices rely on the principle that the force required to make a given depression varies directly with the hardness of the specimen. Such testing devices employ a spherical indenter of known diameter which bears against the workpiece to be tested under a specified load, to form an impression or indentation in the surface of the test piece. The hardness of the test piece is directly proportional to the depth of the indentation. It is difficult to accurately measure the penetration depth and thus the hardness measurements are frequently inaccurate.
One such prior art tester is known as the Brinell type tester and measures the depression after a spherical indenter ball has been removed from the specimen. Thus, elastic recovery of the material after removal of the indenter in this type of tester is another factor which adds to the measuring error. The indentations in the various specimens are geometrically dissimilar and this also adds to the measuring error. In addition, if lighter loads are used to provide a depression diameter which is less than 25% of the diameter of the indenting spherical ball, the depression diameter is quite shallow and is difficult to accurately measure. To form a depression diameter which is greater than 50% of the indenter ball diameter, the material passes so far up on the penetrating ball that minor changes in the hardness can no longer be interpreted accurately. The Brinell method is slow and the Brinell apparatus is cumbersome to operate. All of the established devices are slow and difficult to automate.
Another factor which must be considered in measuring hardness with the prior art tester is "skin effect." Due to different cooling rates, temperatures, etc., the "skin" of the material generally has a different hardness than the underlying portion of the material. Moreover, the outer surface of a cast workpiece is generally uneven or irregular due to different cooling rates and temperatures of the casting. If the material being tested is very hard and the skin relatively soft, the penetrating ball will easily penetrate the soft skin layer, but will make a very little indentation in the underlying hard material. If the thickness of the skin layer constitutes a substantial portion of the overall depression, the instrument will erroneously read "soft."
Prior to conducting a hardness test the the prior art testing devices the surface or skin material must generally be removed by grinding, for example, and this further slows the prior art testing procedures.
Another prior art hardness testing device utilizes an electrical control circuit having an electrical circuit path which includes a portion of the workpiece. This type of testing apparatus will test only electrically conductive workpieces.
An object of the present invention is to provide a hardness tester which will measure hardness with improved accuracy.
Another object of the present invention is to provide hardness testing apparatus which will enable the rate of testing to be increased.
Another object of the present invention is to provide a hardness testing apparatus for accurately measuring the hardness of a workpiece having an irregular surface.
It is another object of the present invention to provide hardness testing apparatus which will measure hardness by measuring the force or load required to provide a predetermined indentation in the material.
Another object of the present invention is to provide a hardness testing machine of the type described which provides a geometrically similar, constant-depth depression in various materials with variable loads being exerted on the penetrator.
Still another object of the present invention is to provide hardness testing apparatus which will measure the hardness of electrically conductive and non-conductive workpieces.
A further object of the present invention is to provide apparatus of the type described incorporating a stop which will positively interrupt movement of the penetrator into the test specimen after it has penetrated the workpiece to a predetermined depth.
Yet another object of the present invention is to provide hardness testing apparatus of the type described which will individually determine the load on the indenter when it has penetrated the test piece to a predetermined depth.
Another object of the present invention is to provide hardness testing apparatus of the type described which will individually determine the loads exerted on the workpiece penetrator and the stop.
Another object of the present invention is to provide hardness testing apparatus of the type described which will indicate whether the hardness for a test specimen falls within a predetermined range only after the load on the stop reaches a predetermined level.
Other objects and advantages of the present invention will become apparent to those of ordinary skill in the art as the description thereof proceeds.